The present invention relates in general to packaging prepared from a paperboard laminate. More particularly, the present invention relates to a heat-sealable paperboard laminate, and packages constructed from that laminate, which includes a buried polyvinyl alcohol copolymer (PVOH) barrier material.
Barrier materials are used in paperboard packaging to accomplish several results. First, barrier materials are required to prevent the egress from the package of flavors, aromas and other ingredients of the packaged product. Secondly, barrier materials are also required to prevent the ingress into the package of oxygen, moisture and other contaminants that might degrade the packaged product.
Many attempts have heretofore been made to provide barrier properties to paperboard packaging. For example, low density polyethylene (LDPE) is a well known component of prior art paperboard packaging since it provides good moisture resistance, and, because it is heat sealable, it provides a means for fabricating the packages. Likewise, the presence of a metallic foil as an inner barrier also significantly reduces the transmission of flavors and aromas out of the package and the transmission of oxygen into the package. However, laminates including metallic foil are difficult to recycle and the use of foil significantly increases the cost of the resulting package. Other attempts at providing barrier protection in paperboard packaging have involved the use of polymeric barrier materials such as ethylene vinyl alcohol copolymers (EVOH); polyvinylidene chloride and its copolymers (PVDC); polyacrylonitrile and its copolymers (PAN); polyamides (PA); polyethylene terephthalate (PET); polyvinyl chloride (PVC); and polypropylene (PP). Of these materials, EVOH is the preferred barrier material (see article entitled, "HIGH BARRIER POLYMERS", by A. L. Blackwell, 1986 Coextrusion Seminar, Marriott Hilton Head, Hilton Head, S.C., published by TAPPI Press). In addition polyvinyl alcohol (PVOH) has been suggested in the past as a potential barrier material (see U.S. Pat. Nos. 4,239,826; 4,254,169; 4,407,897; 4,425,410; and 4,464,443). However, the patented uses for PVOH as a barrier material are for film-only packaging. This is partly due to the fact that PVOH is highly sensitive to moisture, but its absence from paperboard packaging is believed to be primarily due to the fact that it is difficult to process.
The barrier properties and particularly the oxygen permeability rate of most polymers is dependent to some degree on the relative humidity to which they are exposed. For example, the oxygen permeability of both EVOH and PVOH is lower under dry conditions than under humid conditions, while the oxygen permeability of amorphous nylon (SELAR PA), is lower under humid conditions than under dry conditions. Because of this sensitivity to moisture, most laminates used for packaging which incorporate a barrier, are usually multi-layered, with the barrier material surrounded by layers designed to keep it isolated from both atmospheric humidity and the moist contents of the packaged products. In the case of refrigerated liquid products stored in paperboard containers, both the inside and outside of the container may be at or near 100% RH. If it is assumed that the entire structure is at equilibrium, it may be concluded that the barrier layer, even though sandwiched between other layers, is also at 100% RH. However, for packaging dry products, and non-refrigerated liquid products, where the moisture conditions are less extreme, the moisture sensitivity of the barrier material may not be of overwhelming concern. Thus, the relatively low oxygen permeability of PVOH, particularly at low RH, makes it an attractive candidate for use as a barrier material in paperboard laminates, particularly for packaging non-refrigerated liquid products and for dry products, despite its processing difficulties.